As evaporators of refrigeration devices such as refrigerators and refrigerated showcases, heat exchangers are in use which comprise a heat exchanger finned tube and formed in a zigzag shape in its entirety by bending the finned tube at a plurality of finless portions thereof. The finned tube comprises a hairpin tube, and a plurality of fin groups which are arranged on two straight tube portions of the hairpin tube longitudinally thereof at a spacing and each of which comprises a plurality of parallel plate fins extending across and fixed to the two straight tube portions.
Such heat exchangers have heretofore been fabricated by the following two processes.
The first of the processes is as follows. First prepared are two straight tubes, a multiplicity of plate fins each having two holes, and a tube enlarging device comprising a wire and a tube enlarging ball attached to one end of the wire. The two tubes are then inserted through the respective holes of each plate fin to thereby arrange the plate fins in parallel into a plurality of fin groups as spaced apart on the tubes longitudinally thereof. The wire of the tube enlarging device is subsequently inserted at the other end thereof through each tube and pulled at the other end to force the ball through the tube to enlarge the tube and fixedly fit the plate fins of each fin group around the tube. The two tubes are then welded, each at one end thereof, to opposite ends of a U-shaped bend to thereby interconnect the two tubes by the bend, whereby a heat exchanger finned tube is produced. The finned tube is thereafter bent into a zigzag form in its entirety at portions thereof having no fin groups. In this way, a heat exchanger is fabricated.
Studies are recently under way for the use of hydrocarbon refrigerants which are less likely to destroy the ozone layer and to influence global warming, in refrigerators, refrigerated showcases and like refrigeration devices as substitutes for chlorofluorocarbon refrigerants. Since the hydrocarbon refrigerants are flammable, there is a need to diminish the leakage of the refrigerant.
The heat exchanger fabricated by the first conventional process nevertheless has the following problems. Since the finned tube has seams between the U-shaped bend and the component tubes welded thereto, the refrigerant is likely to leak from the seam portions. Further in the case where the finned tube has, for example, inner fins arranged on its inner surface circumferentially thereof at a spacing and extending longitudinally of the tube so as to give an increased heat transfer area to the tube for an improved refrigeration efficiency, the enlarging ball is more likely to collapse the inner fins to result in increased resistance to the flow of refrigerant and impaired refrigeration performance if the inner fins have an excessive height. Accordingly, the inner fins can not be given an increased height and are less effective for an increase in the heat transfer area, hence a limitation to the improvement in refrigeration efficiency.
Heat exchangers for use as evaporators in refrigerators and refrigerated showcases are fabricated by the second process to be described below. First prepared are a hairpin tube, a multiplicity of plate fins each having two holes which are spaced apart, and a tube enlarging device comprising a pressure rod and an enlarging mandrel attached to one end of the rod. The two straight tube portions of the hairpin tube are then inserted through the respective holes of each plate fin to thereby arrange the plate fins in parallel into a plurality of fin groups as spaced apart on the tube portions longitudinally thereof. The mandrel of the tube enlarging device is subsequently forced into the straight tube portions from each open end of the hairpin tube to enlarge the tube portions and to fixedly fit the plate fins of each fin group around the tube portions of the hairpin tube, whereby a heat exchanger finned tube is produced. The finned tube is thereafter bent into a zigzag form in its entirety at portions thereof having no fin groups. In this way, a heat exchanger is fabricated.
The heat exchanger produced by the second process has no seams in the hairpin tube of the finned tube, so that no leakage of the refrigerant occurs unlike the heat exchanger obtained by the first process. However, the heat exchanger produced by the second process also has the following problem. In the case where the finned tube has, for example, inner fins arranged on its inner surface circumferentially thereof at a spacing and extending longitudinally of the tube so as to give an increased heat transfer area to the tube for an improved refrigeration efficiency, the enlarging mandrel is more likely to collapse the inner fins to result in increased resistance to the flow of refrigerant and impaired refrigeration performance if the inner fins have an excessive height. Accordingly, the inner fins can not be given an increased height and are less effective for an increase in the heat transfer area, hence a limitation to the improvement in refrigeration efficiency.
In order to prevent the inner fins from collapsing, therefore, it is thought useful to enlarge the hairpin tube in its entirety by introducing a pressure fluid into the tube in the second process. In this case, however, the circumferential wall of the tube wrinkles in portions thereof having no fin groups, deforming the tube longitudinally thereof to vary the length of the finned tube and failing to afford a heat exchanger of desired dimensions. The heat exchanger finned tube is bent at a plurality of finless portions thereof, whereas the tube is likely to collapse when bent if wrinkles or creases develop in such portions. Further before the hairpin tube is enlarged, the straight tube portions of the tube are not restrained in any way of course at the finless parts thereof or at the portions thereof provided with the fin group, so that the application of the pressure fluid for the enlargement of the tube involves the problem of greatly deflecting the straight tube portions over the entire length thereof. Additionally, the hairpin tube is likely to rupture at finless portions when enlarged.
An object of the present invention is to overcome the foregoing problems and to provide a heat exchanger which is capable of exhibiting the desired refrigeration performance with the leakage of refrigerant diminished.